Deinking of newsprint by flotation method · newsprint, the ink must be removed from the fibre and separated from the pulp stock. This process is known as deinking [2]. 1.2. The principles

DEINKING OF NEWSPRINT BY FLOTATION METHOD

Bimo Ariadi, B.Sc. (Hons.)

,

A thesis submitted in fulfilment of the

requirements for the degree of

Master of Science

at the University of Tasmania

Department of Chemistry

University of Tasmania

July, 1995

1 - 3

Rotation removes particles that are too small to be removed by screens and

cleaners and yet are too big to be removed by washing. Washing is most efficient at

removing the smallest particles of ink. The optimum size range for the different

techniques is illustrated in Figure 1.1.

Figure 1.1. Optimum particle-size range for the various techniques in ink separation (after Shrinath et. al. [9])

1.2.2.1. Washing and flotation

In contrast to screening and cleaning, which are merely physical separation

processes, washing and flotation operations require chemicals to help them perform

efficiently.

Although washing and flotation are both carried out to remove ink particles,

their operating principles are entirely different. Washing systems are most efficient at

removing ink particles smaller than 104m, while flotation works best at removing

particles in the 10-1004m range. Furthermore, the washing process requires ink

particles to remain in the aqueous phase so that they can be removed along with the

Deinking of Newsprint: An Overview

DEINKING OF NEWSPRINT BY FLOTATION METHOD

Bimo Ariadi, B.Sc. (Hons.)

A thesis submitted in fulfilment of the

requirements for the degree of

Master of Science

at the University of Tasmania

Department of Chemistry

University of Tasmania

July, 1995

This thesis contains no material which has been accepted for the award

of any other higher degree in any tertiary institution.

To the best of my knowledge, this thesis contains no material previously

published, except where due reference is given in the text.

Bimo Ariadi

Authority of access

This thesis may be made available for loan and

limited copying in accordance with

the Copyright Act 1968

Abstract

There is much current interest in development of processes which lead to .

greater utilisation of secondary fibres in papermalcing operations, both in Australia and

overseas. The removal of ink from paper (deinking) is a major step in these

processes. After repulping, ink can be removed from aqueous suspension by a

number of techniques, one of which is flotation. Most commercial deinking facilities

use flotation as the principle method of ink removal.

Studies have been made on the effects of flotation conditions, feedstock

composition, and surfactant during flotation deinking of newspaper (ONP) and

magazines (OMG). Type of surfactant and amount of surfactant appear to affect

deinking performance. Temperature, pH, and furnish also appear to affect deinking

efficiency of the various surfactants investigated. There is an optimum pH of 8.5 for

flotation deinking of a 70/30 mixture of ONP/OMG using a fatty acid type deinking

surfactant.

Increasing proportions of magazines (ash content of 26%) in the feedstock

results in a deinked pulp with higher brightness. However, it was found that the

higher brightness attained is largely due to the addition of higher brightness materials

from the magazines, rather than a more efficient mechanism of ink removal from the

ONP.

Addition of Ca2+ in the pulping stage at low level of addition seem to

improve the brightness response for deinking of newspaper with fatty acids. High

level of addition of Ca2+ seems to have detrimental effect.

An attempt is made to explain the results in terms of a model describing the

flotation deinking process and the interactions occurring between surfactant molecules,

ink particles, fibres, and air bubbles.

II

Acknowledgements

I would like to express my sincerest appreciation for the guidance and

supervision provided by my supervisors Dr Lawrie Dunn, Dr Karen Stack and

Associate Professor John Abbot throughout the entire course of this degree.

I am appreciative of the help provided by Mr Peter Dove (mechanical workshop)

and Mr John Davis (electronic workshop) in assembling the deinking experimental

apparatus.

Many thanks are extended to the Central Science Laboratory staff at the

University of Tasmania, especially Dr Noel Davies for doing the gas chromatography

and mass spectroscopy analysis.

My appreciation is extended to the Research Group at Australian Newsprint

Mills - Boyer Mill for allowing me to use their image analysis equipment.

Many thanks also go to the staff and fellow post-graduate students in the

Department of Chemistry at the University of Tasmania for creating enjoyable and

rewarding experiences during my study.

I would like to specially thank Mrs Margaret Eldridge for the proofreading of

this thesis.

The financial support provided by the Indonesian Institute of Sciences is

greatfully acknowledged.

I would like to express my deepest gratitude to my parents for their continuous

support and encouragement. Last but not least, I would like to thank my wife Windra.

Without her love, encouragement and support, none of this would have been possible.

Contents

Abstract

Acknowledgements ii

CHAPTER 1 DeinIcing of Newsprint: An Overview 1 - 1

1.1. Introduction 1 - 1

1.2. The principles of deinking 1 - 1

1.2.1. Detachment of ink from fibre 1 - 1

1.2.2. Ink removal from stock 1 - 2

1.2.2.1. Washing and flotation 1 - 3

1.3. Printing inks 1 - 6

1.3.1. Types of inks 1 - 6

1.4. The chemistry of deinking 1 - 8

1.4.1. Flotation chemistry 1 - 8

1.4.2. Chemical reaction/mechanism in deinking 1 -9

1.4.2.1. Fibre Swelling 1 - 9

1.4.2.2. Saponification 1 - 10

1.4.2.3. Wetting 1- 11

1.4.2.4. Emulsification/Solubilisation 1 - 11

1.4.2.5. Sequestration/Precipitation 1 - 12

1.4.2.6. Anti-redeposition 1 - 12

1.5. Deinlcing chemicals 1 - 12

1.5.1. Sodium hydroxide 1 - 13

1.5.2. Hydrogen peroxide 1 - 13

1.5.3. Silicates 1 - 14

1.5.4. Chelating agents 1- 14

1.5.5. Surfactants 1 - 15

1.5.5.1. Surface and interfacial tension 1- 16

1.5.5.2. Critical micelle concentration 1- 16

1.5.5.3. Hydrophilic-lipophilic balance 1 - 18

1.5.5.4. Surfactant foaming 1 - 19

1.6. Objectives of the study 1 -20

References 1 - 20

CHAPTER 2 Experimental 2- 1

2.1. Laboratory scale flotation deinlcing method 2 - 1

2.1.1. Stock preparation and reagents 2 - 1

2.1.2. Pulping 2 - 2

2.1.3. Flotation 2 - 3

2.2. Measurement of brightness and colour 2 - 3

2.2.1. Handsheets preparation 2 - 3

2.2.2. Brightness measurements 2 - 3

2.2.3. Measurement of colour by L*, a*, b* system 2 - 4

2.3. Speck count analysis 2 - 5

2.4. Measurement of surface tension 2 - 5

2.5. Measurement of water hardness 2 - 6

References

CHAPTER 3 Conditions in Rotation Deinlcing 3 - 1

3.1. Introduction 3- 1

• 3.2. Effects of NaOH addition and flotation pH 3 - 2

3.2.1. Effects of varying NaOH addition in pulping stage 3 - 2

3.2.2. Effects of flotation pH 3 - 9

3.3. Effects of temperature 3 - 15

3.4. Summary 3-24

References 3 - 24

CHAPTER 4 Feedstock Composition in Flotation Deinking 4 - 1

4.1. Introduction 4 - 1

4.2. Deinking of offset printed newspaper and magazines 4 - 2

4.2.1. DeinIcing of offset printed newspaper 4 - 2

4.2.2. DeinIcing of magazines 4 - 4

4.3. Effects of OMG on the deinlcing of ONP 4 - 6

4.4. Summary 4-12

References 4 - 12

CHAPTER 5 Surface Active Agents in Flotation Deinking 5 - 1

5.1. Introduction 5- 1

5.2. Multi-component synthetic surfactants 5 - 2

5.2.1. Effects of surfactant addition 5 - 2

5.2.2. Effects of pH 5 - 9

5.3. Model fatty acid surfactants 5-11

5.3.1. Effects of chain length 5 - 11

5.3.2. Effects of addition of Ca 2+ in pulping stage 5 - 15

5.4. Summary 5-22

References 5 - 22

CHAPTER 6 Conclusions 6 - 1

1 - 1

Chapter 1


1 .1. Introduction

Paper is one of the man-made substances that is universally used in everyday

life, from newspapers, magazines, books, stationery, posters, tissue to packages, and

many more. Predominantly, paper is produced from wood. However, over the last

100 years, recycled fibre has been an important source of paper making fibre,

particularly for packaging grade paper. In response to current environmental issues,

government legislation, and the market demands for paper containing recycled fibre,

there is much current interest in development of processes which lead to greater

utilisation of recycled fibres in paper-making operations both in Australia [1] and

overseas. A large proportion of paper based material is recycled without the removal

of ink. However, for paper grade requiring relatively high brightness, such as

newsprint, the ink must be removed from the fibre and separated from the pulp stock.

This process is known as deinking [2].

1.2. The principles of deinking

The deinking system consists of two characteristic steps. They are: (i) ink

detachment from fibre and (ii) ink removal from stock

1.2.1. Detachment of ink from fibre.

The first stage in the deinlcing process is referred to as pulping or repulping.

Wastepaper pulping is a relatively simple process, being achieved by supplying water,


1 - 2

heat, chemicals, and mechanical energy. The main aims of repulping are to break

down the wastepaper into discrete fibres and to separate the ink from the fibres [3].

Pulping is a critical operation in deinlcing because in this stage ink is removed

from the fibre. Removal of ink particles from the fibre stock suspension is possible

only if the ink particles are entirely detached from the fibres prior to entry into the

separation stage.

Chemicals are normally added to the pulper just prior to the addition of

furnish. Stock consistencies are usually between 4 and 6%; however there is a trend

towards higher consistency (12-15%) pulping because of the savings in chemicals,

heat, and other operational costs [4]. High alkalinity and temperature (50°C) are

beneficial [5].

The amount of mechanical energy generated by the pulper is important in

determining the rate of defibering and the rate of ink removal and dispersion. This

mechanical energy is dependent upon the pulper configuration and the pulping

consistency. However, the chemicals added to the pulper are the primary determinant

of the level of ink dispersion and will be discussed later.

1.2.2. Ink removal from stock

After the ink is detached from the fibre, it must be removed from the stock.

This is accomplished by a number of techniques, such as screening, cleaning,

washing, and flotation. The size of the ink particles to be removed is the primary

basis for choosing the appropriate technique.

Screens and centrifugal cleaners are used to remove large particles of ink.

Particle size and shape do have some influence on ink removal by centrifugal cleaners,

with larger particles (100-1000w) being more effectively removed [6,7]. Ink

removal by screening is poor because the flat ink particles tend to align themselves

with the fibres and pass through the screen [8].


1 - 3

Flotation removes particles that are too small to be removed by screens and

cleaners and yet are too big to be removed by washing. Washing is most efficient at

removing the smallest particles of ink. The optimum size range for the different

techniques is illustrated in Figure 1.1.

Figure 1.1. Optimum particle-size range for the various techniques in ink separation (after Shrinath et. al. OD

1.2.2.1. Washing and flotation

In contrast to screening and cleaning, which are merely physical separation

processes, washing and flotation operations require chemicals to help them perform

efficiently.

Although washing and flotation are both carried out to remove ink particles,

their operating principles are entirely different. Washing systems are most efficient at

removing ink particles smaller than lOwn, while flotation works best at removing

particles in the 10-1004m range. Furthermore, the washing process requires ink

particles to remain in the aqueous phase so that they can be removed along with the


Washing

** Hydrophilic

'particles

Add dispersant and inorganics

Wash fibre

1 - 4

water. In contrast, flotation relies on the capture of ink particles by air bubbles, which

rise to the surface, forming a foam that can be skimmed off as rejects. Ink particles

separated by flotation must be rendered hydrophobic so that they can be easily

separated from the water phase and attach themselves to air bubbles. Figure 1.2

depicts the mechanism involved in washing and flotation, which use different

chemicals to accomplish their objectives.

Figure 1.2. Comparison of washing and flotation (after Horacek and Jarrehutt DOD

In washing, it is necessary to keep the ink particles finely dispersed and

prevent their agglomeration [11]. To achieve this, dispersants are used. Washing also

requires the ink particles to be rendered hydrophilic so they remain in the aqueous

phase.

For flotation to be effective, the size of the ink particles must be maintained

within the optimum range. The particles also must be hydrophobic. Particles that are


Yield loss 15 - 20% 5 - 10%

Fillers and fines Removed Retained

Conc. sludge Ink removed Very dilute

Slightly lower Chemical costs

Somewhat lower Capital costs

Higher strength Higher opacity Pulp quality

Waste water Some in situ treatment Must be done externally

1 - 5

too small are not efficiently collected because of the low probability of encountering air

bubbles. Very large particles are likely to be too bulky to be successfully carried to the

surface by the bubbles. Hydrophobic particles are more easily separated from the

aqueous phase and carried to the surface by air bubbles.

Table 1.1. illustrates the main differences of washing and flotation deinlcing.

One implication of this comparison is that flotation is preferable for printing papers

since fines and fillers are acceptable, but for products such as tissue, washing may

offer some advantages [12].

Table 1.1. Comparison of washing and flotation (after Sauzedde [12D

To take advantage of the benefits of both technologies, most new deinking

plants will be a combination washing/flotation system. However, because of the

conflicting operating principles of washing and flotation, chemicals that aid one

process can hinder the other.


1 - 6

1 . 3 . Printing inks

Since deinking deals with ink removal, it is essential to understand a little

about printing inks. This section will discuss printing inks, especially those that are

commonly used in newspaper and magazines.

Printing ink has two basic ingredients which are:

(i) Pigments, which provide the proper contrast to the image area and

provide colour and opacity to the ink.

(ii) Vehicle, which carries the pigments and helps transfer the pigment to

the sheet and aids in binding it there. Vehicles are generally vegetable

oils, mineral distillates, and resins (natural and synthetic).

Printing inks also can contain several other components including binders,

solvents, dryers, wetting agents, and waxes. The make up of the ink is determined by

the type of paper on which it is to be applied, the method of application (printing

process), the drying process, and the end use of the paper.

1.3.1. Types of inks

Ink is frequently classified according to its setting method. The general ink-

setting methods (listed in Table 1.2) are absorption, evaporation, oxidation, and

radiation curing [13, 14].

The absorption method is used with inks containing oil in the vehicle. The oil

is absorbed by pores in the paper, leaving the pigment behind on the paper surface.

This method is usually used in newsprint

The evaporation method is used with inks containing volatile solvent vehicles

that evaporate and cause the ink to dry. Vehicles are typically rosin esters or metal

resinate binders dissolved in a suitable solvent. This method is used in letterpress and

web offset printing for magazines and catalogues and in rotogravure printing of

newspaper supplements.


:4estog'utothocti:;::: - Fogsodtinkorna... "

•Hydrocarbon (mineral) •Hydrocarbon resins

•Not subject to saponification

•Vehicle must be emulsified and/or mechanically dispersed

Absorption

• Hydrocarbon solvent •Rosin esters or metal

resinate • Hydrocarbon resins •Alkyd resins and

oleoresinous varnishes

•Rosin esters difficult to saponify

•Metallic resinates saponifiable

•Hydrocarbon resins need to be emulsified and/or dispersed

Evaporation

•High boiling hydrocarbons

•Oil-modified alkyds •Oleoresinous varnishes •Phenolic-modified rosin

esters

•Polymerised films not soluble in common solvents

•Partially saponified with strong alkali at elevated temperature

Oxidation

•Epoxy acrylates •Polyol acrylates •Urethane acrylates •Photo initiators - (aryl

ketones)

•Not saponifiable •Chemical dispersion

difficult •Not soluble in common

solvents

Radiation curing

1 - 7

The oxidation method is a combination of absorption and polymerisation of

the oil or resin in the vehicle. The result is a polymerised film that is more flexible and

tougher than films formed by the evaporation method. Both web and sheet fed offset

printing processes use this technique.

The radiation curing method involves application of radiation to polymerise

the ink. Radiation curing is used in high-gloss protective coating magazines and

specialty products.

Table 1.2. Ink-setting method (after Scarlett [13] and Bassemir [14])


CH3 (CH2)x— C \ 0 Na+

Fatty acid component

(Hydrophobic end)

Functional group

(Hydrophilic end)

1 - 8

1.4. The chemistry of deinking

1.4.1. Flotation Chemistry

The removal of ink particles by flotation is a physical-chemical process. It is

based on the phenomenon that separation is achieved by influencing the wettability,

with water, of the particles to be separated. The water-repellency of the surface of the

particles to be separated is achieved by addition of special hetero-polar chemicals

which deposit on the surface of the particles.

The course of the entire flotation process can be influenced by the variation of

physical and chemical factors. Physical variables include the ink particle size and

density, the size of the air bubbles, the consistency and temperature of the pulp slurry

or suspension, as well as the velocity and flow conditions in the flotation cell. Among

the chemical variables are the quality of the water (eg., water hardness), the pH value

of the pulp slurry or suspension, and the flotation agents, such as collector and

frothers.

Soaps, the alkali salts of fatty acids, with a long chain of molecules

containing a hydrophobic (fatty acid) group at one end and a hydrophilic (functional)

group at the other (Figure 1.3) are the best known collectors in flotation deinlcing [15].

Figure 1.3. Example for a surface-active substance (soap)


1 - 9

The simplified reaction mechanism of ink flotation proposed by Ortner

et.al.[16] is shown in Figure 1.4. The surface-active substances, which reduce

surface tension, lead to the formation of froth on the water-air boundary. The

hydrophobic ends face the ink particles, and the hydrophilic ends are directed towards

the water. As a result, the enveloped ink particles appear hydrophilic on the outside

and detach more easily from the fibre(1.4b). To deposit the dispersed ink particles

(1.4c) on air bubbles (1.4a), the hydrophilic ends of the soap molecules must react

with hardening constituents of water (eg., Ca 21 so that they act as collectors (1.4d).

The soaps precipitated by the hardening constituents of the water act as collectors,

while the non-precipitated soaps are effective as frothers and dispersing agents.

1.4.2. Chemical reaction/mechanism in deinking

In flotation deinlcing, prior to flotation stage, it is very important that ink is

sufficiently broken up and dispersed in the pulper. Chemicals are added to enhance

this process. The addition of chemicals causes a number of complex reactions to

occur. Some of the general reactions and mechanisms involved will be discussed.

1.4.2.1. Fibre Swelling

This process begins as soon as the wastepaper is immersed in water. The

water molecules form hydrogen bonds with the cellulose molecules and break

interfibre hydrogen bonds. The effect is enhanced with the addition of caustic and

elevated temperature [17]. The breaking of interfibre bonds and swelling of fibres are

an important part in deinking, as they greatly facilitate loosening and removal of ink

particles and coatings from fibre surfaces.


1 - 10

Figure 1.4. Ink flotation model (after Ortner et.al. (a) air bubbles stabilised by frother. (b) ink particles detaching from fibres. (c) dispersed ink particles. (d) ink particles whose surface-active substance (soap) has reacted with hardening constituents and which now deposit on air bubbles. (e) foam laden with ink particles.

1.4.2.2. Saponification

Saponification is a chemical reaction that proceeds under alkaline conditions

to convert an ester to its component alcohol and salt (soap). The equation for the

saponification reaction is as follows:

0 0 II II

R—C-0—R + NaOH R—C 0- Na+ + R—OH

Ester Alkali Soap Glycerol (vegetable oil) (sodium salt of ester)


Many of the resins used as ink binders are esters and therefore can be broken

up in hot alkali solutions. This is one of the principle reactions occurring in high pH

deinlcing of conventional offset and gravure inks. The oily vehicle in standard

newsprint ink is similar to hydrocarbons; therefore, it is not subject to saponification.

This is also the case for the modified hydrocarbon resins often used in offset inks.

Phenolic modified rosin esters can be saponified under severe conditions of pH and

temperatures.

1.4.2.3. Wetting

This is a surface or interfacial phenomenon that plays a key role in pulping

liquor penetration into the fibre network. When a liquid surface is in contact with a

solid, the molecules at the interface may be more attracted to the solid than to the bulk

liquid. If so, the molecules tend to spread out over the solid and the surface area of

the liquid is increased. This phenomenon is called wetting. Proper wetting allows

more rapid penetration of chemicals into the fibre network and inter-fibre contact area

and helps inks break up and separate from fibres.

1.4.2.4. Emulsification/Solubilisation

Emulsification is the dispersion of one liquid phase into another to form a

significantly stable suspension. Similar to wetting, emulsification is a surface

phenomenon that requires addition of surfactants to alter the interfacial tension

between the phases. Emulsification is an important chemical mechanism in deinking

only when there are oils present in the ink These inks are used in letterpress and

offset printing of newspaper and magazines, and they dry primarily by absorption.

Adsorption of emulsifying agents (surfactants) at the oil/fibre interface releases the oil

from the fibre (with the pigment particles) and forms an oil in water emulsion.

Solubilisation is the dissolving of substances in a medium in which they are

normally insoluble. Solubilisation differs from emulsification in that in the former the


1 - 12

solubilised material is in the same phase as the solution while the latter is a dispersion.

Solubilisation may be the most important mechanism for the removal of oily inks, as it

has been observed [18] that removal of oily soil from textile surfaces becomes

significant only under conditions that favour solubilisation.

1.4.2.5. Sequestration/Precipitation

The presence of polyvalent cations - notably calcium, magnesium, and iron -

can be detrimental to the deinking process even, to a certain extent, when nonionic

surfactants are used. These cations can reduce negative surface charges on both fibre

and ink [19] leading to agglomeration and redeposition, and cations also may act as

linkages between negatively-charged fibres and negatively-charged ink particles.

These ions enter the system in the water or paper stock and can be removed by

sequestration (formation of a water-soluble complex) and precipitation (formation of

an insoluble precipitate).

1.4.2.6. Anti-redeposition

Anti-redeposition refers to the prevention of suspended particles from

precipitating onto the substrate from which they were removed. In deinking, the

dispersed ink particles must be kept from settling back onto the fibres. Anti-

redeposition is achieved by sterically inhibiting the approach of ink particles to fibres.

1 . 5 . Deinking chemicals

Chemicals for deinking are chosen based on wastepaper, ink types, and

design of the deinking system (washing or flotation). Also of importance is the

quality of stock going to the paper machine. Many mills are incorporating various

percentages of deinIced fibre into their final product The amount of deinked stock that

may be blended with virgin fibre is largely determined by the quality of the deinked

stock. For a specific deinking system that quality may be controlled by careful use of


1 - 13

deinlcing chemicals. The following sections will discuss common deinking chemicals

in some detail.

1.5.1. Sodium hydroxide

Sodium hydroxide (NaOH), also referred to as caustic soda, is one of the

most important deinking chemicals for wood-free secondary fibre, as well as for

deinking wood-containing furnishes such as newsprint. High concentrations of alkali

can saponify and/or hydrolise many ink binders and will swell fibres, aiding in the

breaking up of inks and coatings.

However, addition of sodium hydroxide to wood-containing furnishes will

cause the pulp to yellow and darken. This is a phenomenon often referred to as alkali

darkening. It was reported that the effect of pH on the formation of chromophores in

lignin increase as the pH rises above 5.5 [20].

1.5.2. Hydrogen peroxide

Hydrogen peroxide (H202) is used to decolourise the chromophores

generated by the alkaline pH in a wood-containing furnish. The peroxide reaction

with sodium hydroxide is as follows:

H202 + NaOH H00- + H20

The bleaching action of hydrogen peroxide is attributed to the oxidative action

of the perhydroxyl anion (HOO') [21,22]. To maximise the amount of perhyciroxyl

anion (HOO') peroxide bleaching is normally carried out under alkaline conditions.

However, it is also well known that hydrogen peroxide has a tendency to decompose

according to the following equation:

H202 1/2 02 + H20


1 - 14

which is known to be catalysed by the presence of heavy metal ions like manganese,

copper, and iron, high pH and temperature [23,24,25].

Hydrogen peroxide decomposition can be reduced by addition of small

amounts of the pentasodium salt of diethylenetriaminepentaacetic acid (Na5DTPA)

[23]. Sodium silicate is also an important additive in peroxide bleaching of mechanical

pulps. Evidence indicates that silicate does not stabilise peroxide by itself but

stabilises the environment within which the peroxide works [26].

1.5.3. Silicates

Silicates have been used since the turn of the century in deinking wastepaper.

It was reported that silicates, compared to caustic soda alone, provide better ink

removal and brighter pulps with less fibre damage [27].

Silicates are complex solutions of polymeric silicate anions which are surface

active. This surface activity is what gives silicates many of their deinlcing functions,

including emulsification and suspension of dispersed ink [28].

Sodium silicate is a good stabiliser for alkaline peroxide bleaching solutions.

Silicates tend to decrease the rate of peroxide decomposition by inactivating heavy

metal catalysts present in the bleaching solution [29,30,31]. Silicates are used

primarily in deinking newsprint or other high goundwood containing furnishes.

1.5.4. Chelating agents

DTPA (diethylene triarnine penta-acetic acid) is the most commonly used

chelant. The role of chelant is to form soluble complexes with heavy metal ions [26].

The structure of DTPA is shown in Figure 1.5. The complexes prevent the heavy

metal ions from decomposing the hydrogen peroxide. The metals can be sourced from

the wastepaper or from the water. DTPA will chelate metals in the following order of


N—CH2CH2 CH2CH2—N

Na0OCH2C/ \

N/ \CH2COONa

CH2COONa

Na0OCH2C\ /CH2COONa

1 - 15

priority [32]:

Ni2+ > cu2+ > co2+ > Fe2+ > mn2+ > pb2+ >

Zn2+ > Fe3+ > Ca2+ > Mg2+ > A13+

Figure 1.5. The structure for Na5DTPA

1.5.5. Surfactants

Surfactants are surface active substances that contain an organic part that has

an affinity for oils (hydrophobe) and another part that has an affinity for the water

phase (hydrophile). The hydrophobic group is usually a long chain hydrocarbon

residue while the hydrophilic group is ionic or, in the case of nonionic surfactants, a

highly polar group. These surfactants function in deinlcing systems by lowering the

surface tension of water to enable it to "wet" more effectively, adsorbing onto surfaces

to aid in ink removal and dispersion, and by solubilisation and emulsification.

Surfactant chemistry and its practical application are complex. There are

thousands of available surfactants whose function and performance are influenced by

many application conditions. Blends of surfactants will provide better performance

than single components [18].

Several surfactant properties play a significant role in determining surfactant

effectiveness. They include surface tension and interfacial tension, critical micelle

concentration (cmc), hydrophilic-lipophilic balance (HLB), and surfactant foaming.


1 - 16

1.5.5.1. Surface and interfacial tension

Interfacial tension is defined as the work required to increase the area of an

interface isothermally and reversibly by a unit amount In speaking of the liquid-air

interface, it is general practice to use the term surface tension.

As has been mentioned before the surface-active molecules are characterised

by the presence of a polar and a non-polar group. The polar portion of the molecule is

surrounded by a strong electromagnetic field and exhibits a high affinity for other

polar groups and molecules, including water. The non-polar portion of the molecule

has a low affinity for water and other polar molecules. The surface energy of a liquid

or a solution depends on the potential energy of the electromagnetic field which

extends outwards from the surface layer atoms. For the surface energy, to have a

minimum value, it is necessary that the molecules present in the solution arrange

themselves so that the least active portions of the various species present in the

solution are exposed at the surface [33]. Thus, for solutions of surfactants in water,

the surfactants molecules will tend to concentrate at the surface, with the non-polar

portion directed outwards (Figure 1.6). This arrangement provides the minimum

contact between the water molecules and the non-polar hydrocarbon chain of the

surfactant molecule, thereby reducing the solution surface tension.

Surface tension is an important concept in surfactant chemistry. It can be

conceptualised as a force per unit length at a right angle to the force required to pull

molecules apart to expand the surface area [18]. Therefore, a liquid with low surface

tension spreads more.

1.5.5.2. Critical micelle concentration

The concentration of surfactant at which the concentration of micelles

suddenly becomes appreciable is referred to as the critical micelle concentration (cmc).

The surface activity, in general, is due to non-micellar surfactants and the micelles act

as a reservoir for the unassociated surfactants molecules and ions. At a concentration


1 - 17

greater than the cmc value, the surface tension of the solution does not decrease further

with an increase in surfactant concentration, since surfactant molecules are forming

micelles in the bulk of the solution instead of packing the surface of the liquid (Figure

1.6).

Hydrophope Surfactant molecule

HydrOphile

Air

III 1111111 II 111 11 111 Surfactant concentration below CMC. Surfactant collects at air-water interface, does not form micelles.

Water

o

Increase surfactant concentration

alga inifirat atet)

I • trt, oir Or Mrent......4,

MP OM OM QM MP

Il■ ..... OW is .., . en mm.4 SIMEMP mm. M. ........ .... "'. .......1. ...7"" '. I - ------

A

S

Surfactant concentration above CMC. Surfactant forms micelles, solubilises oils from fibres.

0--

cr-

Water

Figure 1.6. The schematic diagram of surfactants in solution (after Borchardt [34])


oil in water emulsifier 8 - 18

13 - 15 detergency

15 - 18 solubilising agent

water in oil emulsifier

wetting agent 7 - 9

4 - 6

1 - 18

The ability of surfactant solutions to dissolve or solubilise water-insoluble

materials, so as to remove ink from fibre, is due to the formation of micelles. Micelles

are clusters of surfactant molecules in which the hydrophiles are oriented towards the

water phase. The hydrophobes are oriented away from the aqueous phase and

towards the interior of the micelle (Figure 1.6). This creates an oil-like environment in

the interior of the micelle. Oils such as ink vehicles are solubilised when they are

taken into the interior of surfactant micelles.

1.5.5.3. Hydrophilic-lipophilic balance

One way of characterising surfactants is by their hydrophilic-lipophilic

balance or HLB. This concept was developed by Griffin [35,36] and is based on the

fact that any surface active agent contains both hydrophilic and lipophilic groups, and

the ratio of their respective weight percentages should influence their dispersive and

emulsifying behaviour. A low HLB indicates a surfactant that is lipophilic in

character, while a high HLB indicates one that is hydrophilic in character. Table 1.3

illustrates the application of surfactants as related to HLB value.

Table 1.3. Surfactant application as related to HLB value (after Griffin [351)


Oriented double layer of molecules

in stabilised bubble L 4.• -_11.1/111 -

-

--- - . •. .... _ _ ...... ..........._____.__ -___ ....... .....•. ..• ----- - --- - - —....... 1 : -- : .... — --• :

...----...73:: Itt/i.

ti.

r: ••• :

Z Air _ . Air bubble

- with adsorbed - - - surfactant

: - - - - - - :

Air

44\

wiligilum* Foam bubble

- - _ -41111411M11

• " - = = ! 4--3 • - - . . . .. .

..... 01 ,w rao 4S%.• ..... •___ _ E

No ••.'• ... . ..............

41/11.1111° ....C;•: .■ ..... • qw. ••

1 - 19

Turai and Williams [37] have done some experiments on the role that HLB

has on deinlcing efficiency. In their work, they found that in the deinlcing of

newsprint the optimum HLB value for nonionic surfactant is in the range 14.5 to 15.5.

1.5.5.4. Surfactant foaming

The foaming process is depicted in Figure 1.7. Surfactant molecules orient

themselves around an air bubble with the hydrophobe pointing towards the bubble and

away from the aqueous pulp slurry. The surfactant hydrophile groups are oriented

towards the aqueous phase. Since an air bubble is less dense than water, it rises to the

top of the pulp slurry. If foam generation is faster than the rate that air bubbles break

open, a foam layer builds up.

Figure 1.7. Surfactant stabilisation of air bubbles and foam formation (after Borchardt DO

In flotation deinking, foaming separates the ink particles from the pulp slurry

and traps them in a froth layer. Since, even below the cmc, some surfactant molecules

congregate at the air-water interface (as indicated in Figure 1.6), foaming can occur


1 - 20

below the cmc. Thus, surfactants may be used in a flotation cell below the critical

micelle concentration.

1 . 6. Objectives of the study

The main objectives of the study on the deinlcing of newsprint by flotation are

to define the general principles for the behaviour of deinIcing chemicals, in particular

the surfactants, under flotation deinking conditions and also to gain an understanding

of the surface chemistry phenomena in the deinking of newsprint, particularly those

that control the efficiency of deinlcing.

The study will investigate the effects of flotation conditions such as pH and

temperature, feedstock composition, and surfactant during flotation deinIcing of

newspaper (ONP) and magazines (0MG). The results of these experiments will be

explained in terms of a model describing the flotation deinking process.

References

1. Higgins, H.G. - Tappi Journal, 75(3): 99 (1992)

2. Woodward, T.W. - Pulp and Paper, 60(11): 59 (1986)

3. Welsford, J. - Proceedings of the Jaalcko Payry/Appita Deinking Conference,

Melbourne, 1991

4. Barassi, J. and Welsford, J. - Appita, 45(5): 308 (1992)

5. Merrett, KJ. - Appita, 40(3): 185 (1987)

6. Serres, A. and Levis, S.H. - Proceedings of the CPPA Technical Section, 1991

7. Julien, F. and Perrin, B. - Pulp and Paper Canada, 94(10): 29 (1993)

8. Harrison, A. - Pulp and Paper, 63(3): 60 (1989)

9. Shrinath, A., Szewczalc, T. and Jerry Bowen, I. - Tappi Journal, 74(7): 85

(1991)


1 - 21

10. Horacek, R.G. and Jarrehult, B. - Pulp and Paper, 63(3): 97 (1989)

11. Fergusson, L.D. - Tappi Journal, 75(8): 49 (1992)

12. Sauzedde, C. - Proceedings of the Jaaldco Payry/Appita DeinIcing Conference,

Melbourne, 1991

13. Scarlett, T. - Proceedings of the Tappi Pulping Conferences, p. 181, 1981

14. Bassemir, R.W. - Proceedings of the Tappi Annual Meeting, p. 99, 1982

15. Schweizer, G. - Wochenblatt fuer Papierfabrikation, 93(19): 823 (1965)

16. Ortner, H., Wood, R.F. and Gartemann, H. - Wochenblatt fuer

Papierfabrikation, 103(16): 597 (1975)

17. Sjostorm, E - Wood Chemistry Fundamentals and Applications, Academic

Press, New York, Chapter 9, 1981

18. Rosen, MJ. - Surfactants and Interfacial Phenomena, Wiley-Interscience, 1978

19. Larsson, A., Stenius, P. and Odberg, L. - Svensk Papperstidning, 87(18):

R158 (1984)

20. Andrews, D.H. and Singh, R.P. - The Bleaching of Pulp, Tappi Press, Atlanta,

p.211-253, 1979

21. Slave, M.C. - Tappi Journal, 48(9): 535 (1965)

22. Teder, A. and Tormund, D. - Svensk Papperstidning, 83(4): 106 (1980)

23. Colodette, J.L., Rothenberg, S. and Dence, C.W. - J. Pulp Paper Science,

14(6): J126 (1988)

24. Lachenal, D., de Choudens, C. and Monzie, P. - Tappi Journal, 63(4): 119

(1980)

25. Kutney, G.W. and Evans, T.D. - Svensk Papperstidning, 88(9): R84 (1985)

26. Colodette, J.L., Rothenberg, S. and Dence, C.W. - J. Pulp Paper Science,

15(1): J3 (1989)

27. Falcone, J.S. and Spencer, R.W. - Pulp and Paper, 49(14): 114 (1975)


1 - 22

28. Ali, T., McLellan, F., Adiwinata, J., May, M. and Evans, T. - J. Pulp Paper

Science, 20(1): (1994)

29. Ali, T., McArthur, D., Scott, D., Fairbank, M. and Whiting, P. - J. Pulp Paper

Science, 12(6): J166 (1986)

30. Fairbank, M.G., Colodette, J., Ali, T., McLellan, F. and Whiting, P. - J. Pulp

Paper Science, 15(4): J132 (1989)

31. Ali, T., Fairbank, M., McArthur, D., Evans, T. and Whiting, P. - J. Pulp Paper

Science, 14(2): J23 (1988)

32. Mathur, I. - Pulp and Paper Canada, 94(10): T310 (1993)

33. Langmuir, I. - Phenomena, Atoms and Molecules, Philosphical Library Inc.,

New York, 1950

34. Borchardt, J.K. - Progress in Paper Recycling, 2(1): 45 (1992)

35. Griffin, WJ. - J. Soc. Cosmetic Chem., 1: 311 (1949)

36. Griffin, WJ. - J. Soc. Cosmetic Chem., 5: 249 (1954)

37. Turai, L.L. and Williams, L.D. - Tappi Journal, 60(11): 167 (1977)


2 - 1

Chapter 2

Experimental

2 . 1 . Laboratory scale flotation deinking method

This section describes the method used for flotation deinlcing. The equipment

used is a laboratory scale Lamort Deinlcing Unit (see Figure 2.1), which can be set up

in the pulper (Figure 2.1.a) or flotation (Figure 2.1b) arrangement. This unit enables

one to duplicate industrial operating conditions of a pulper or a deinking unit with

relatively small quantities of waste paper.

2.1.1. Stock preparation and reagents

Old newspaper(ONP) was obtained in batches of recently printed offset

Mercury newspaper (2-3 months old) from The Mercury Press in Hobart, while a

range of magazines (OMG) was obtained, also in batches, from the Angus and

Robertson Bookstore in Hobart. A selection of magazines (eg. Cleo, Women's

Weekly etc.) was taken as representative of coated magazines. The age of these

magazines was approximately 6-12 months.

Different batches of ONP and OMG were used for the experiments during the

course of this study. Brightness from different batches of ONP and OMG varied

slightly (by 1 -2 unit).

The newspaper and magazines were separately torn into 2-3 cm squares. All

staples and glue from bindings were removed prior to pulping, and the samples were

stored in opaque plastic bags.

Sodium hydroxide (98%) and hydrogen peroxide (30%) were obtained from

BDH chemicals. DTPA (97%) and fatty acids were obtained from Aldrich. Sodium

Experimental

2 - 2

silicate (30%) was provided by Aluminates Chemicals, Burnie, Tasmania. The

deinlcing surfactant samples, designated as surfactants A, B, C, and D, were supplied

by Buckman Laboratories.

Figure 2.1. Schematic diagram of Lamort Deinking Unit in (a) pulper and (b) flotation arrangement.

Experimental

2 - 3

2.1.2. Pulping

In the pulping stage the Helico Pulper was installed into the Lamort deinlcing

unit. Pulping was carried out using 750g o.d. fibre at 8% consistency. Hot water at

50°C and chemicals (1% NaOH, 1% sodium silicate, 1% H202, 0.2% DTPA, and

0.4% deinking surfactants) were introduced into the pulper before the waste paper.

Once a good rolling/mixing action was achieved, pulping was continued for 20

minutes.

2.1.3. Flotation

For flotation, the Lamort Hyperflotation Kit replaced the Helico Rotor,

incorporating an aeration screen and air suction column on the rotor and installing an

overflow weir for ink sludge collection. A 450 g o.d. sample of the repulped stock

was used for flotation. The stock was diluted with water to fill up the tank. In some

instances the rotor was also used as a mixer for adjusting the pH of the slurry, before

putting the aeration screen in place. Flotation was performed at a consistency of 1%

and temperature of 50°C. The temperature and pH of the pulp slurry in the tank

remained quite stable during the flotation.

2.2. Measurement of brightness and colour

2.2.1. Handsheet preparation.

Handsheets were formed to evaluate the repulped and post-flotation (deinked)

stock. The stock was acidified to pH 5 prior to sheet formation, to simulate the pH

shock deinked pulp would experience prior to being used in acidic paper-making

conditions. The method used in preparing the handsheet is similar to that described in

TAPPI Official Methods (T218 om-91) [1] using No. 41 filter paper. The filter paper

was removed before drying. Three handsheets were prepared for each experiment.

Experimental

2 7 4

2.2.2. Brightness measurement

The term brightness refers to the reflection of light at a wavelength of 457

rim. This particular wavelength measurement gives good correlation with what the eye

appreciates as brightness. Brightness readings were made using an Elrepho 2000

Spectrophotometer (Figure 2.2) at 457nm. Measurements were taken on both sides of

the sheets and reported as an average.

Figure 2.2. The Elrepho 2000 spectrophotometer.

2.2.3. Measurement of colour by L*, a*, b* system

The L*, a*, b* system locates the colour in a colour solid. The diagram for

the L*, a*, b* system is given in Figure 2.3. The L* coordinate represents the

lightness from black at the bottom through a series of greys to white at the top. The a*

coordinate goes from green to red and the b* coordinate from blue to yellow. The L*,

a*, b* colour scales are expressed in National Bureau of Standards units of colour

difference. The magnitude of this unit is such that one unit represents about the

maximum colour difference that an observer will tolerate in an average commercial

Experimental

2 - 5

colour match. The L*, a*, b* values were also measured using the Elrepho 2000

Spectrophotometer.

Figure 2.3. Rectangular dimensions of the L*, a*, b* solid for designating the colours of surfaces (after Casey [2D.

2 . 3 . Speck count analysis

A speck count of the handsheet made from deinked pulp was made using an

image analysis facility at Australian Newsprint Mills Research Division at Boyer,

Tasmania [31. The equipment for image analysis consisted of an IBM PC/XT

computer, two display monitors and a Panasonic video camera. Used in conjunction

with this are a Pentax macro lens and a light source consisting of two incandescent

globes. The basic image analysis software consists of a package called Visilog.

The image analysis equipment detects ink particles which are greater than 65

imn and its measurement area is 512 x 512 pixel (where 1 pixel is equal to 65 urn).

The image analysis equipment was calibrated with a grey tile which has an ISO

brightness of 53%. In speck count analysis, any specks in the handsheet which are

darker than 53% brightness were detected by the image analysis software and the area

of the specks calculated in parts per million or mm 2 of specks per m2 of handsheet.

Erperimental

2-6

2.4. Measurement of surface tension

Surface tension measurements were made using an Analite surface tension

meter (Figure 2.4). The surface tension meter consists of a movable platform which is

raised or lowered by means of a fine height control mechanism, a thin glass plate and a

plate holder. These components are located in a working area which is enclosed by

sliding glass doors to prevent draughts and reduce sample contamination.

Figure 2.4. Analite Surface Tension Meter.

For measuring surface tension, the fluid sample to be measured is placed in a

container on the platform, and the platform is raised until the glass plate is fractionally

immersed in the fluid. The surface tension is indicated directly on the digital readout

in millinewtons per meter when using the standard glass plates which are supplied

with the meter.

2.5. Measurement of water hardness

Water hardness measurements were made using a Univer Water Hardness

Kit, which consists of EDTA titrant (0.35N), Univer hardness reagent powder

Experimental

2 - 7'

pillows, plastic measuring tube (5.83 mL) and square mixing bottle. For measuring

water hardness, one full measuring tube of water sample to be measured was

transferred to a square mixing bottle and mixed with the contents of one Univer

hardness reagent powder pillow. The mixture then was titrated against EDTA until the

colour changes from red to blue. The water hardness of the sample was expressed in

mg/L of hardness as calcium carbonate (CaCO3).

References

1. Tappi Test Methods - Tappi Official Method T218-om91, Tappi Press, Atlanta,

1991

2. Casey, J.P. - Pulp and Paper, Vol. ifi, Third edition, John Wiley and Sons,

1981

3. Collins, NJ. and Rosson, AJ. - Appita, 41(11): 475 (1988)

Experimental

3 - 1

Chapter 3

Conditions in Flotation Deinking

3.1. Introduction

Alkalinity is known to affect the peroxide bleaching reaction. Alkaline

chemicals react with hydrogen peroxide to form the perhydroxyl ion, H00 - , which is

instrumental in the bleaching action of the hydrogen peroxide [1,2]

H202 + OW -II." H20 + H00 - However, it is also widely known that high alkalinity in wood-containing feedstock

will cause the pulp to yellow and darken. It was shown that the formation of

chromophores in lignin rapidly increase as the pH rises above 5.5 [1]. This is a

phenomenon often referred to as alkali darkening.

Sodium hydroxide (NaOH), also referred to as caustic soda, is used in

deinking formulation to adjust the pH to the alkaline region, pH 9.5 - 11,

conventionally employed in pulping [3]. The presence of alkali causes the fibre to

swell and opens up fibrils, thus helping the detachment of ink particles from the

fibres. Alkali partially breaks down vegetable oil ink vehicles by saponification and

controls chemical species in solution (such as DTPA) by pH. Hence, it is important to

know the effects of alkalinity in flotation deinking. This chapter aims to investigate

the effects of NaOH addition and flotation pH on efficiency of deinldng as measured

by brightness at 457nm, and colour using the L*, a*, b* scales. The effects of

temperature will also be discussed.

The experiments that were carried out in investigating the effects of alkalinity

and temperature employed several different surfactants as listed in Table 3.1.

Conditions in Rotation Deinking

3 - 2

Table 3.1. Type of deinking surfactants used (supplied by Buckman Laboratories)

. ig.tfat ... . ./..0. . :dAigtAiRiPSONO ::.. „.,„Maitem. .ponentx. . ,...... 4. ;§::::::::::::::::::::::::::::::, A Combination of fatty acids and a non-ionic

surfactant.

B Combination of fatty acid soaps, a non-ionic surfactant, and ethylene glycol.

C Combination of ciirnethylamide of C18 oil, non-ionic surfactant, anionic surfactant, dipropylene glycol methyl ether, and aromatic solvent.

3.2. Effects of NaOH addition and flotation pH

3.2.1. Effects of varying NaOH addition in pulping stage

Caustic soda (NaOH) is one of the main ingredients in deinking formulation.

The addition rate of NaOH in industry is usually stated in terms of percentage on

ovendry fibre. The following experiments were done to investigate the effects of

varying the initial amount of NaOH applied at the pulping stage, keeping other

variables at constant values. The deinking feedstock that is used is the 70/30 mixture

of offset printed newspaper and coated magazines. This feedstock composition is

chosen because it is the composition that is generally accepted as a desirable feedstock

for the deinking process in industry, as both are available in large quantities.

The efficiency of deinking is measured in terms of brightness at 457 nm as

well as colour in the L*, a*, b* scales, as some researchers [4] have stated that colour

measurements made with the L*, a*, b* colour scale are preferable to brightness

measurements made at 457 nm as a means of accurately representing ink removal.

Figures 3.1 to 3.4 present the results of the effect of varying the initial

amount of NaOH applied at the pulping stage on deinlcing of 70/30 mixture of offset

printed newspaper and coated magazines by using the deinking surfactant A, which is

predominantly a mixture of saturated and unsaturated fatty acid soaps.


3 - 3

70

Bri

ght

ness

(%

ISO

) 65 -

60 -

55 -

• Repulped • Deinked

50

0.0

0.5 1.0

1 . 5

2 . 0

% NaOH

Figure 3.1. Effect of NaOH addition (as % o.d. fibre) on brightness of pulp after pulping and flotation for deinking of 70/30 mixture of ONP/OMG using surfactant A. Pulping conditions: 1% H202, 1% sodium silicate, 02% DTPA, 0.4% surfactant A; time 20 mins; temperature 50°C; consistency 8%. Flotation conditions: time 6 mins; temperature 50°C; consistency 1%.

Figure 3.1 shows an increase in brightness after pulping with increasing

%NaOH applied (indicated by empty dots). It also illustrates that there seems to be an

optimum %NaOH (or more likely pH) for the brightness after flotation (indicated by

filled dots). The apparent decrease above 1% NaOH for the brightness after flotation

is probably due to alkali darkening. The decrease in brightness after pulping above

1% NaOH is not apparent, probably due to the existence of perhydroxyl ion at

sufficient concentration to counter the yellowing or chromophore creation due to high

alkalinity. At the flotation stage, the pulp consistency is diluted to 1% consistency,

hence the concentration of perhydroxyl ion in terms of molarity is also reduced by

approximately a factor of 8. Meanwhile, pH of the pulp slurry remains high at

pH > 9.5 (Table 3.2).


95

90 -

L 85 -

80 -

. • Repulped • Deinked

75

3 - 4

Table 3.2. The correspondent pH values associated with the initial amount of NaOH applied.

0 . 0

0 .5 1.0

1 . 5

2 . 0

% NaOH Figure 3.2. Effect of NaOH addition (as % o.d. fibre) on L* of pulp after pulping and flotation for deinking of 70/30 mixture of ONP/OMG using surfactant A. Pulping and flotation conditions as in Figure 3.1.

The L* values (in Figure 3.2), which are a measure of greyness, show similar trends

to the brightness measurements at 457 nm (Figure 3.1). It shows increasing L*

values after pulping with increasing %NaOH applied, with diminishing rate of

increase as %NaOH increased


3 - 5

a*

-4-


-6

0.0 0.5

1.0

1 . 5

2 . 0

% NaOH

Figure 3.3. Effect of NaOH addition (as % o.d. fibre) on a* of pulp after pulping and flotation for deinking of 70/30 mixture of ONP/OMG using surfactant A. Pulping and flotation conditions as in Figure 3.1.

b•

10

8

6

4

2

0 0.0

0.5

1.0

1 . 5

2 . 0

% NaOH

Figure 3.4. Effect of NaOH addition (as % o.d. fibre) on b* of pulp after pulping and flotation for deinking of 70/30 mixture of ONP/OMG using surfactant A. Pulping and flotation conditions as in Figure 3.1.


65

t----'-------.-

60 -

55 ."

o Repuiped • Deinked

50 —

•

3 - 6

There seems to be very little change in a* values, which are an indication of

the green-red tint of paper, with change in %NaOH after pulping. The value of a*

remains constant at -0.6 after pulping (Figure 3.3).

Figure 3.4 shows that, after pulping, b* values, which are an indication of

the yellowness of the paper, increase with %Na0H. It is apparent that b* values after

flotation are higher than those after pulping, indicating that the yellowing reaction is

occurring further in the flotation stage, where it is favoured by the high alkalinity and

low concentration of perhydroxyl ion present in the flotation stage.

Figures 3.5 to 3.8 present the results of the effect of varying the initial

amount of NaOH applied at the pulping stage on deinlcing of 70/30 mixture of offset

printed newspaper and coated magazines by using the deinlcing surfactant C, which is

predominantly a mixture of saturated and unsaturated dimethylamides of a C18 oil.

0.0 0.5 1.0

1.5

2.0

% NaOH Figure 3.5. Effect of NaOH addition (as % o.d. fibre) on brightness of pulp after pulping and flotation for deinking of 70/30 mixture of ONP/OMG using surfactant C. Pulping conditions: 1% H202, 1% sodium silicate, 0.2% DTPA, 0.4% surfactant C; time 20 mins; temperature 50°C; consistency 8%. Flotation conditions: time 6 mins; temperature 50°C; consistency 1%.

Brig

htne

ss (

%IS

O)


1.5 2.0 0.0

0.5 1.0

% NaOH

3 - 7

O Repulped • D einked

-6

Figure 3.6. Effect of NaOH addition (as % o.d. fibre) on L* of pulp after pulping and flotation for deinking of 70/30 mixture of ONP/OMG using surfactant C. Pulping and flotation conditions as in Figure 3.5.

a*

0.0

0.5 1.0

1.5

2.0

% NaOH

Figure 3.7. Effect of NaOH addition (as % o.d. fibre) on a* of pulp after pulping and flotation for deinking of 70/30 mixture of ONP/OMG using surfactant C. Pulping and flotation conditions as in Figure 3.5.


3 - 8

b*

10

8

6

4

2

0 0.0

0.5 1.0 1.5 2.0

% NaOH

Figure 3.8. Effect of NaOH addition (as % o.d. fibre) on b* of pulp after pulping and flotation for deinking of 70/30 mixture of ONP/OMG using surfactant C. Pulping and flotation conditions as in Figure 3.5.

Repeating the experiment by using a different surfactant (surfactant C)

generally shows similar tends as those observed before (surfactant A), although they

differ in the extent they increase or decrease with variation in the initial amount of

NaOH.

The increase in the brightness of pulp after pulping with increasing initial

amount of NaOH is still observable (Figure 3.5), suggesting that alkali conditions are

necessary in the pulping stage for both surfactant A and C.

It seems also there is a maximum in the brightness response after flotation

with increasing initial amount of NaOH applied, where application of NaOH higher

than 1% favours the yellowing reaction in the flotation stage.

It is generally known that in peroxide bleaching there is competition between

two reactions as alkalinity changes. They are: (i) chromophore removal or peroxide

bleaching, and (ii) alkaline darkening or chromophore formation. The presence of a

maximum in brightness after flotation reflects this competition. Above 1% application


3 - 9

of NaOH, alkali darkening predominates over chromophore removal. The value of b*

(Figure 3.8) appears to be more affected by the alkali darkening reaction that is

favoured by high pH.

3.2.2. Effects of flotation pH

In an attempt to get more insight into the effects of alkalinity in flotation

deinking, it was decided to do some experiments where the pH in the flotation stage

was adjusted and the initial amount of NaOH applied in the pulping stage was fixed.

The application of 1%NaOH in the pulping stage was chosen to ensure that there was

enough alkalinity for the pulping and ink detachment from the fibre to be successful.

The experiments were done on deinking of 70/30 mixture of offset printed

newspaper and coated magazines by using the deinlcing surfactants A and C. In the

flotation stage, pH of the pulp slurry was adjusted to a desired value by the addition of

NaOH or HC1.

The plots in Figures 3.9 to 3.16 show the changes in brightness and colour,

L* a* b* values, for the pulp after flotation as well as after the pulping stage. Since

there are no process variable changes made in the pulping stage, it is expected that the

response will remain constant (represented by the empty dots). They are plotted in the

graph for reference for the changes observed in the response after flotation stage due

to pH changes, which are represented by filled dots.


Bri

ght

ness

(%

IS0)

3-10

2

4

6 8

10

12

Flotation pH

Figure 3.9. Effect of pH adjustment prior to flotation stage on brightness of pulp for deinking of 70/30 mixture of ONP/OMG using surfactant A. Pulping conditions: 1%Na0H, 1% H202, 1% sodium silicate, 0.2% DTPA, 0.4% surfactant A; time 20 mins; temperature 50°C; consistency 8%. Flotation conditions: time 6 mins; temperature 50°C; consistency 1%.

2 4 6 8

12

Flotation pH

Figure 3.10. Effect of pH adjustment prior to flotation stage on L* of pulp for deinking of 70/30 mixture of ONP/OMG using surfactant A. Pulping and flotation conditions as in Figure 3.9.


3- 11

a*

4

2

0

-2-

-4 -


•

-6

2

4

6

8

10

12

Flotation pH

Figure 3.11. Effect of pH adjustment prior to flotation stage on a* of pulp for deinking of 70/30 mixture of ONP/OMG using surfactant A. Pulping and flotation conditions as in Figure 3.9.

b*

Flotation pH

Figure 3.12. Effect of pH adjustrnent prior to flotation stage on b* of pulp for deinking of 70/30 mixture of ONP/OMG using surfactant A. Pulping and flotation conditions as in Figure 3.9.


Bri

ght

ness

(%

IS0)

90

o


85 -

80 -

75 1

3-12

• I I • I 1 • 2

4 6 8

10

12

Flotation pH

Figure 3.13. Effect of pH adjustment prior to flotation stage on brightness of pulp for deinking of 70/30 mixture of ONP/OMG using surfactant C. Pulping conditions: 1%Na0H, 1% H202, 1% sodium silicate, 0.2% DTPA, 0.4% surfactant C; time 20 mins; temperature 50°C; consistency 8%. Flotation conditions: time 6 mins; temperature 50°C; consistency 1%.

L*

2 4 6 8

10

12

Flotation pH

Figure 3.14. Effect of pH adjustment prior to flotation stage on 12' of pulp for deinking of 70/30 mixture of ONP/OMG using surfactant C. Pulping and flotation conditions as in Figure 3.13.


b*

. I • I 2 4 6 8

Flotation pH

1 0 12

3-13

4

2

0

V

-2

-4 -

• Repulped • Deiniced

-6

2

4

6 8

10

12

Flotation pH

Figure 3.15. Effect of pH adjustment prior to flotation stage on a* of pulp for deinking of 70/30 mixture of ONP/OMG using surfactant C. Pulping and flotation conditions as in Figure 3.13.

Figure 3.16. Effect of pH adjustment prior to flotation stage on b* of pulp for deinking of 70/30 mixture of ONP/OMG using surfactant C. Pulping and flotation conditions as in Figure 3.13.


3 - 14

The results once again illustrate that b* value is increasing with increasing

pH (as shown in Figures 3.12 and 3.16), which gives further evidence that the b*

value or the yellowness of the pulp is significantly affected by pH. Higher pH in the

flotation stage favours the chromophore formation, which is reflected in the higher b*

values.

The results, in Figures 3.9 and 3.13, indicate that the two surfactants behave

differently as pH is varied. The brightness responses illustrate that there is an

optimum pH at which the brightness response is at a maximum. The presence of an

optimum pH is more noticeable for surfactant A (Figure 3.9). Up to pH -8.5, the

brightness increases as the flotation pH is increased. Above pH -8.5, further increase

in flotation pH results in a decline in the brightness response. For surfactant C

(Figure 3.13), the brightness response is fairly constant up to pH -8.5, above which

the brightness response starts to decline slightly. The loss of brightness at higher pH

can be partly accounted for by the increasing yellowing reaction or chromophore

formation, as illustrated by higher b* values. However, it is unlikely that yellowing

reaction or chromophore formation is solely responsible for brightness loss,

considering the different response for different surfactant type. It is possible for pH to

affect the performance of surfactants in ink removal process. However, the

mechanism of how pH could affect the performance of surfactants could not be

explained due to lack of information on the exact composition of the surfactants.

The response of L* value for surfactant A, in Figure 3.10, generally follows

that of brightness (Figure 3.9), where L* value increases with increasing flotation pH

up to -8.5 and then decreases beyond pH -8.5. In the case of surfactant C, the

response of L* value seems to decrease slightly with increasing flotation pH (Figure

3.14). This might suggest that, since L* values represent the greyness of the pulp and

therefore ink removal [5], flotation pH hardly affected the performance of surfactant C

in the ink removal process. However, further evidence is required to support this

statement.


3-15

The a* values for both surfactants (Figures 3.11 and 3 .15) are hardly

affected by changes in flotation pH over the pH range studied.

3. 3. Effects of temperature

The effects of temperature in both the pulping and flotation stages were also

investigated and the results are shown in Figures 3.17 to 3.20 for surfactant A and

Figures 3.21 to 3.24 for surfactant C. Figures 3.17a and 3.21a show the effect on

brightness upon varying the temperature in the pulping stage (maintaining the flotation

temperature at 50°C), and Figures 3.17b and 3.21b show the effect on brightness

upon varying the temperature in the flotation stage (maintaining the pulping

temperature at 50°C). The results clearly show that there is an enhancement of

brightness response associated with increased flotation temperature in the range 20 0 to

50°C. The benefit on increasing the temperature through this range corresponds to an

improvement of -3 brightness units (Figures 3.17b and 3.21b). In contrast,

increasing the temperature in the pulping stage (maintaining the flotation temperature at

50°C) has little significant impact (Figure 3.17a) or no effect (Figure 3.21a) on the

brightness achieved.

The L* value, as shown in Figure 3.18 and 3.22, gave rise to similar trends

as did the brightness measurements at 457 nm. However, for surfactant C,

temperature increase in the pulping stage (maintaining the flotation temperature at

50°C) has slightly more impact on the L* value (Figure 3.22a) than on the brightness

(Figure 3.21a). The yellowness of the pulp as represented by b* value generally

increased as temperature increased (Figures 3.20 and 3.24). There was very little

change in the a* value (Figures 3.19 and 3.23) with temperature change.

The different responses of the surfactants A and C toward temperature

changes suggest that the performance of surfactant is affected by temperature. The

nature and chemical composition of the surfactants is responsible for this different

behaviour. It was suggested by some researchers that for surfactant to be effective in


3-16

flotation the surfactant molecules should form aggregates [6]. Increasing temperature

favours the surfactant molecules coming more closely together and forming

aggregates.


Bri

ghtn

ess

(%I S

O)

Bri

ghtn

ess

( %IS

O)

3 - 17

1 20

30 40 50

60 Pulping Temperature (°C)

10 20 30 40 50

60 Flotation Temperature (°C)

Figure 3.17. Effect of temperature during (a) pulping with constant flotation temperature of 50°C, and (b) flotation with constant pulping temperature of 50°C on brightness for deinking of 70/30 mixture of ONP/OMG using surfactant A. Pulping conditions: 1%Na0H, 1% H202, 1% sodium silicate, 0.2% DTPA, 0.4% surfactant A; time 20 mins; consistency 8%. Flotation conditions: time 6 mins; consistency 1%, pH 8.5.


1 I • I 20

30 40 50

60

Pulping Temperature (°C)

I 3-18

95

90 -

1

1

I • I • I • I 10 20 30 40 50

60

Flotation Temperature (°C)

Figure 3.18. Effect of temperature during (a) pulping with constant flotation temperature of 50°C, and (b) flotation with constant pulping temperature of 50°C on L* for deinking of 70/30 mixture of ONP/OMG using surfactant A. Pulping and flotation conditions as in Figure 3.17.


-6 1 i I

4 (a)

2

0 - e

-2-

o Repulped • Deinked

-4-

3-19

a*

60 20 30 40

50


4 (b)

2

0

-2

-

0

-4- • Repulped • Deinked

-6 10

I • I . 20 30

I • I 40 50 60


Figure 3.19. Effect of temperature during (a) pulping with constant flotation temperature of 50°C, and (b) flotation with constant pulping temperature of 50°C on a* for deinking of 70/30 mixture of ONP/OMG using surfactant A. Pulping and flotation conditions as in Figure 3.17.


10

8 -

6 -

4 -

2 - o Repulped • Deinked

(a)

0 .

3 - 20

b*

20 30 40 50

60 Pulping Temperature (°C)

b*

- 10

20 30 40 50


Figure 3.20. Effect of temperature during (a) pulping with constant flotation temperature of 50°C, and (b) flotation with constant pulping temperature of 50°C on b* for deinking of 70/30 mixture of ONP/OMG using surfactant A. Pulping and flotation conditions as in Figure 3.17.


a 60 -

55 -

Brig

htne

s s

(%IS

O)

(b) 65

60 -

55 -

50 -

o Repulped • DeinIced

45

3-21

65 (a)

50 -

O Repulped • Deinked

45 20

30 40 50

60


10 20 30 40 50

60


Figure 3.21. Effect of temperature during (a) pulping with constant flotation temperature of 50°C, and (b) flotation with constant pulping temperature of 50°C on brightness for deinking of 70/30 mixture of ONP/OMG using surfactant C. Pulping conditions: 1%Na0H, 1% H202, 1% sodium silicate, 0.2% DTPA, 0.4% surfactant C; time 20 mins; consistency 8%. Flotation conditions: time 6 mins; consistency 1%, pH 8.5.

Brig

htne

ss (

%IS

O)


90 (a)

85 -

80 - e e

• Reptdped • Deinked

I • I 75 1

3 - 22

L*

2 0 3 0 40 50

60


L*

10 20 30 40 50


Figure 3.22. Effect of temperature during (a) pulping with constant flotation temperature of 50°C, and (b) flotation with constant pulping temperature of 50°C on l..* for deinking of 70/30 mixture of ONP/OMG using surfactant C. Pulping and flotation conditions as in Figure 3.21.

60


4 (b)

111

0

2


-2-

-4-

-6

3 - 23

4 (a)

2

0 r,

a*

-2

-4- • Repulped • Deinked

-6 20 30 40 50


60

a*

10 20 30 40 50

60


Figure 3.23. Effect of temperature during (a) pulping with constant flotation temperature of 50°C, and (b) flotation with constant pulping temperature of 50°C on a* for deinking of 70/30 mixture of ONP/OMG using surfactant C. Pulping and flotation conditions as in Figure 3.21.


3-24

b*

20 30 40 50

60


b*

10 20 30 40 50


Figure 3.24. Effect of temperature during (a) pulping with constant flotation temperature of 50°C, and (b) flotation with constant pulping temperature of 50°C on b* for deinking of 70/30 mixture of ONP/OMG using surfactant C. Pulping and flotation conditions as in Figure 3.21.


3 - 25

3.4. Summary

An alkali environment in the pulping stage is crucial to ensure successful ink

detachment from fibres. However, high alkalinity for wood-containing furnishes,

such as 70/30 mixture of offset printed newspaper and coated magazines, can induce a

phenomenon known as alkaline darkening which is due to the formation of

chromophores. To counter the formation of chromophores, hydrogen peroxide is

added in the pulping stage, which is responsible for chromophores removal.

In the flotation stage, pH adjustment in some cases is beneficial in order to

minimise the yellowing of the pulp due to high alkalinity. However, it also has been

illustrated that the performance of the surfactant in ink removal in flotation stage is also

influenced by pH.

Temperature is also an important factor which accounts for variation in

deinking efficiency as measured by brightness. In the range 20° to 50°C, increasing

temperature in the flotation stage (maintaining the pulping temperature at 50°C) is more

beneficial than increasing temperature in the pulping stage (maintaining the flotation

temperature at 50°C).

References

1. Andrews, D.H. and Singh, R.P. - The Bleaching of Pulp, TAPPI Press,

Atlanta, p. 211-253, 1979

2. Strunk, W.G. - Pulp and Paper, 54(6): 156 (1980)

3. Fergusson, L.D. - Tappi Journal, 75(7): 75 (1992)

4. Weiss, G.R., Levis, CJ. and Gupta, R. - Pulp and Paper Canada, 91(8): T316

(1990)

5. Mathur, I. - Pulp and Paper Canada, 94(10): T310 (1993)

6. Wood, D.L. - Proceedings of the Tappi Pulping Conference, Tappi Press,

Atlanta, p. 435, 1992

Conditions in Flotation Dein king

4 - 1

Chapter 4

Feedstock Composition in Flotation Deinking

4.1. Introduction

One of the variables in flotation deinking beside the pulping and flotation

conditions is the deinking feedstock itself. Newspaper and magazines have been

traditionally used as the feedstock for flotation deinking. Variables within the

feedstock paper, such as type of ink, method of printing, paper grade, as well as the

proportion of newspaper and magazines in the feedstock, can all affect the efficiency

of deinking.

The old newspaper (ONP) sample used in this study was offset printed

newspaper. It was about 2-3 months old. The old magazines (OMG) sample was

represented by a selection of magazines (eg. Cleo, Women's Weekly etc.). The age of

these magazines was approximately 6-12 months. The OMG sample has an ash

content of 26%.

The first part of the study on the variation in feedstock composition in

flotation deinlcing aims to investigate the deinicing of newspaper and magazines

individually and study the deinlcing efficiency in terms of brightness and colour (in

L*, a*, b* scales). The second part aims to look at the effect of magazines (OMG)

inclusion in the flotation deinking of newspaper (ONP).

Surfactant sample B was chosen to investigate the variation in feedstock

composition. Surfactant sample B is a multi-component surfactant, its main

components are combinations of C18 fatty acid soaps, similar to surfactant sample A

used to study the effect of pH and temperature in Chapter 3.

Feedstock Composition in Rotation Deinking

80

70 -

60

50 -


40

4 - 2

4. 2 . Deinking of offset printed newspaper and magazines

In this experiment, newspaper (ONP) and magazines (OMG) were deinked

separately. The effects of surfactant addition on deinking efficiency in terms of

brightness, L* and b* were examined.

4.2.1. Deinking of offset printed newspaper

The effect of surfactant addition on deinking of 100% ONP on brightness is

shown in Figure 4.1. Only a small (2 unit) increase in brightness occurs on flotation.

This is not uncommon. This is due to low ink content in newspapers. Also the ISO

brightness of the non-inked areas of ONP is around 60%.

Figure 4.1 shows that above the addition level of 0.6% surfactant B the

brightness response starts to decline. This effect is more noticeable in the brightness

response after pulping.

0.0 0.2 0.4 0.6 0.8

1 . 0

% Surfactant (on o.d. fibre)

Figure 4.1. Effect of Surfactant B addition (as % o.d. fibre) on brightness of pulp after pulping and flotation for deinking of ONP. Pulping conditions: 1% NaOH, 1% H202, 1% sodium silicate, 0.2% DTPA; time 20 mins; temperature 50°C; consistency 8%. Flotation conditions: time 6 mins; temperature 50°C; consistency 1%.

Brig

htne

ss (

%1S0

)

Feedstock Composition in Rotation Deinking

95

90 -

85

80 -

•

9 Repulped • Deinked

70

75 -

o Repulped • De,inlced

4 - 3

L*

0.0 0.2 0.4 0.6 0.8

1.0


Figure 4.2. Effect of Surfactant B addition (as % o.d. fibre) on L* of pulp after pulping and flotation for deinking of ONP. Pulping and flotation conditions as in Figure 4.1.

10

b*

0 0.0 0.2 0.4 0.6 0.8 1.0


Figure 4.3. Effect of Surfactant B addition (as % o.d. fibre) on b* of pulp after pulping and flotation for deinking of ONP. Pulping and flotation conditions as in Figure 4.1.


Bri

ght

nes

s (%

IS0)

70

60

50

40

80

4 - 4

The L* response in Figure 4.2 shows a similar trend to the brightness

response. It is apparent, from Figures 4.1 and 4.2, that there is an optimum range of

surfactant addition level for deinlcing of 100% ONP, above which the brightness and

L* response suffer a loss.

The b* response (Figure 4.3) shows a slight increase as the level of

surfactant addition increases. Figure 4.3 also shows that the b* values of the pulp

after pulping stage are generally the same as those after flotation stage.

4.2.2. Deinking of magazines

The effect of surfactant addition on brightness on deinking of 100% OMG is

shown in Figure 4.4. A 20 unit increase in brightness occurs, which is much higher

than that obtained for 100% ONP (Figure 4.1). This is due to a higher ink content in

OMG than in ONP. Also, the non-inked areas of OMG have a higher ISO brightness

(around 75%) compare to that of ONP.

0.0 0.2 0.4 0.6 0.8

1 . 0


Figure 4.4. Effect of Surfactant B addition (as % o.d. fibre) on brightness of pulp after pulping and flotation for deinking of OMG. Pulping conditions: 1% NaOH, 1% H202, 1% sodium silicate, 0.2% DTPA; time 20 mins; temperature 50°C; consistency 8%. Flotation conditions: time 6 mins; temperature 50°C; consistency 1%.

Feedstock Composition in Flotation Deb:king

4-5


0.0 0.2 0.4 0.6 0.8

1 . 0


Figure 4.5. Effect of Surfactant B addition (as % o.d. fibre) on L* of pulp after pulping and flotation for deinking of OMG. Pulping and flotation conditions as in Figure 4.4.

b*

10

8

6

4

2

0 0.0

0.2 0.4

0.6 0.8

1 . 0


Figure 4.6. Effect of Surfactant B addition (as % o.d. fibre) on b* of pulp after pulping and flotation for deinking of OMG. Pulping and flotation conditions as in Figure 4.4.

95 -

90

85 -

L*

80 -

75

70


4 - 6

Figure 4.4 shows that the brightness responses for the pulp after both

pulping and flotation stages increase as the level of surfactant addition increases. This

is in contrast to the results for deinlcing of 100% ONP where brightness loss was

observed above a certain level of surfactant addition.

The L* response in Figure 4.5 shows a similar trend as the brightness

response. It is interesting to note that, after the pulping stage, the brightness and L*

value for 100% OMG is generally lower than those for 100% ONP. However, after

the flotation stage, the brightness and L* value for 100% OMG is generally higher

than those for 100% ONP (comparing Figure 4.1 to 4.4 and Figure 4.2 to 4.5).

The b* response (Figure 4.6) also shows an increase as the level of surfactant

addition increases. Figure 4.6 also shows that the b* values of the pulp after the

flotation stage are higher than those after the pulping stage. Comparing Figure 4.6 to

4.3, it was apparent that the b* values for 100% OMG, both after pulping and

flotation stage, are generally lower compared to the b* values for 100% ONP.

4.3. Effect of OMG on the deinking of ONP

It is a common practice to use a mixture of newsprint and magazines in

industrial deinking processes. A mixture of 70/30 ONP/OMG is common, and this is

a typical feedstock composition used at the ANM Albury plant in NSW. It is generally

believed that the inorganic fillers introduced with the magazines is beneficial in

promoting deinking [1]. It is also widely believed that an ash loading of 8-10% on

o.d. fibre needs to be maintained in the flotation cell [2]. However, there are few

reported studies of the exact roles which magazine inclusion and filler usage play in

the flotation deinking. This section will study the inclusion of OMG in the flotation

deinking of ONP and attempt to explain the phenomena observed.

The first part will look at the effect of surfactant addition on the deinking

efficiency of 70/30 mixture of ONP/OMG, and the second part will examine the effect

of changing the ONP/OMG rates over a wider range.


80

70 -

60 -

50 -


40

4 - 7

The effect of surfa

Deinking of newsprint by flotation method · newsprint, the ink must be removed from the fibre and separated from the pulp stock. This process is known as deinking [2]. 1.2. The principles

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